The following types of cell are all involved in non-specific cellular defence mechanisms. This means that they will attack a wide range of invading organisms.

For more information on white blood cells see lecture notes for The Blood

Neutrophils (60-70% of WBC count)
Once in the area of infection neutrophils respond to chemicals (called chemotaxins which are released by bacteria and dead tissue cells) and move towards the area of highest concentration. Here they begin the process of phagocytosis in which they engulf the offending cells and destroy them with their powerful enzymes. Because this process consumes so much energy the neutrophils glycogen reserves are soon depleted and they die soon after phagocytosis. When the cells die their contents are released and the remnants of their enzymes cause liquefaction of closely adjacent tissue. This results in an accumulation of dead neutrophils, tissue fluid and abnormal materials known as pus.

This picture shows a neutrophil engulfing a single cell of Candida albicans which is a fungal micro-organism responsible for Thrush in human.

Note how the Candida cell is completely enclosed within a large vacuole inside the neutrophil. The small dark areas on the edge of the vacuole are lysosomes which are about to discharge their contents into the vacuole in order to destroy the fungal cell.

After this process is complete the neutrophil itself will also die.

Eosinophils (1.5%)
They increase greatly in many types of parasitic infection and defence against the larvae of parasitic worms and unicellular organisms seems to be one of their primary functions. The granules of eosinophils contain a substance called MBP (major basic protein) which is toxic to many parasitic larvae. Eosinophils also have surface receptors for the antibody immunoglobulin E (IgE). These receptors are not found in neutrophils and again this is thought to reflect their role in parasitic infection.

Natural Killer Cells (5-8%)
A type of lymphocyte found in secondary lymph organs
They are activated by substances call interleukins and interferons
Recognise infected cells especially those attacked by viral agencies
Destroy body's own cells if invaded
Also have the ability to attack aberrant cells that could cause tumours

Macrophages (~5%) these cells are derived from monocytes.
They are found in various body tissues and remove dead cell debris as well as attacking organisms such as Tubercule Bacilli (which causes TB) and some fungi. Neither of these can be dealt with effectively by the neutrophils. Unlike neutrophils monocytes are able to replace their lysosomal contents and are thought to have a much longer active life.
They are also important in specific immunity as we shall see later.

Mechanisms for killing bacteria

There are many ways in which these cells are able to kill bacteria and other foreign bodies but basically they belong to two categories.

Oxygen dependent
or
Oxygen independent

Oxygen dependent mechanisms

Oxygen dependent mechanisms basically result from the respiratory burst associated with the act of phagocytosis.
This results in the release of two chemicals H2O2 (hydrogen peroxide) and HOCl (hydrochlorous acid)

These are two very powerful chemicals that are able to break down the protective wall of bacteria and other micro-organisms.
Of clinical importance in "chronic granulomatous disease of childhood" a condition in which the enzymes required to produce these enzymes are missing. As a result neutrophils can ingest bacteria but cannot break them down.

Oxygen independent mechanisms

Lysozyme : an enzyme that attacks cell wall of some bacteria (especially Gram+ve) N.B. it is lacking in people with Down's Syndrome and hence accounts for their susceptibility to certain infections

Lactoferrin : a chemical which binds onto iron thus inhibiting cell growth especially in bacteria

Major Basic Protein (MBP) : a cationic protein found in eosinophils principally active against parasitic infections such as roundworms, liver flukes etc.

Bactericidal Permeability Increasing Protein :  as it's name suggests this substance increases permeability of cell membrane of many micro-organisms making them more vulnerable to attack from other agents of the immune system.

In addition to this white blood cells produce a lower pH (more acidic) environment which adds to the anti-bacterial properties.
Unfortunately these mechanisms are not 100% successful.
Most noticeably the Mycobacterium species that cause leprosy and tuberculosis can actually live inside phagocytic cells such as neutrophils and are thus protected against drugs and the specific defence mechanisms.